First, core center is a figment of your imagination...there is no such thing. If you want to be able to ski the whole mountain well, don't go center...they will sink like a mofo in fresh snow...I'd say go 3-4 (cm, not inches) back from center measure it out and draw a line across both skis with a sharpie, tell them to put your midsole right on that line.
And when people refer to mounting + or - from center, it's also in cm, never in inches...+ in forward of the line, - is behind the line.

ON3P
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"Nothing is impossible. You are only limited by fear and even that you can overcome."
-Seth Morrison
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"a simple equation:

force of impact + force of steeze = 0

the two negate each other. for eric, stomping a 60 ft cornice switch is like jumping on a really soft hotel mattress." - ChronicF explaining why Eric Pollard still has knees.